The invention relates to a method and reactor for applying a coating to a fiber by chemical vapor deposition (CVD).
Fiber reinforced ceramic matrix composites (CMCs) are formed of continuous uniaxial or woven fibers of ceramic material embedded in a ceramic matrix. These materials are designed to have a weak fiber-matrix bond strength. When a CMC is loaded above a stress that initiates cracks in the matrix, the fibers debond from the matrix allowing fiber/matrix sliding without fiber fracture. The fibers bridge the matrix crack and transfer load to the surrounding matrix by transferring tensile stress to frictional interfacial shear force.
Conventional manufacturing processes for CMCs have comprised, first, preparing a flowable suspension of the ceramic matrix material in powdered form, coating or immersing the inorganic fibers (usually long fiber tows) with the suspension, and collecting the coated fiber tows into a mat or other shape. The impregnated fibers and shapes made therefrom are at this stage of the process commonly termed “prepreg.” The prepreg can be reshaped as desired but is ultimately formed into a preform for the desired composite article. The preform is then subjected to a burn-out step wherein organic or other fugitive components present in the coating are removed from the preform. The debindered preform is finally consolidated into a dense composite material by reaction with molten silicon at high temperature.
Preferred fibers for producing CMCs are of relatively small diameter (8-15 microns). The fibers are wrapped in tows of several hundred individual filaments. The coating is applied to the fibers for several purposes such as to protect them during composite processing, to modify fiber-matrix interface stresses and to promote or prevent mechanical and/or chemical bonding of the fiber and matrix. Chemical vapor deposition (CVD) is a preferred technique for production of coatings on fibers used for production of ceramic matrix composites.
The coating process must uniformly coat all fibers. While this is possible when the fibers are individual filaments, it is very difficult to achieve when the fibers are collected into tows and woven, braided or knitted into fabrics. Adjacent and neighboring fibers block the flow of gaseous coating reactant to the fiber surfaces and block counterflow of reaction by-product gases away from the fibers. There is a need for an improved CVD process and reactor for uniformly coating CMC fibers.